Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 227

Chapter 17 Configuring STP and MST Understanding How STP Works Extended System ID A 12-bit extended system ID field is part of the bridge ID (see Table 17-1 on page 17-3). Catalyst 6500 series switches have 64 MAC addresses and always use the 12-bit extended system ID. Table 17-1 Bridge Priority Value and Extended System ID with the Extended System ID Enabled Bridge Priority Value Extended System ID (Set Equal to the VLAN ID) Bit 16 Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1 STP MAC Address Allocation Catalyst 6500 series switches have 64 addresses available to support software features such as STP. To view the MAC address range, enter the show catalyst6000 chassis-mac-address command. STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN. With MAC address reduction enabled on any device, you should also enable MAC address reduction on all other Layer 2 connected network devices to avoid undesirable root bridge election and spanning tree topology issues. When MAC address reduction is enabled, the root bridge priority becomes a multiple of 4096 plus the VLAN ID. With MAC address reduction enabled, a switch bridge ID (used by the spanning tree algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. If another bridge in the same spanning tree domain does not run the MAC address reduction feature, it could win root bridge ownership because of the finer granularity in the selection of its bridge ID. Understanding Bridge Protocol Data Units Bridge protocol data units (BPDUs) are transmitted in one direction from the root bridge. Each network device sends configuration BPDUs to communicate and compute the spanning tree topology. Each configuration BPDU contains the following minimal information: • The unique bridge ID of the network device that the transmitting network device believes to be the root bridge • The STP path cost to the root • The bridge ID of the transmitting bridge • Message age • The identifier of the transmitting port • Values for the hello, forward delay, and max-age protocol timers When a network device transmits a BPDU frame, all network devices connected to the LAN on which the frame is transmitted receive the BPDU. When a network device receives a BPDU, it does not forward the frame but instead uses the information in the frame to calculate a BPDU, and, if the topology changes, initiate a BPDU transmission. A BPDU exchange results in the following: • One network device is elected as the root bridge. OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 17-3